Sorry but no, not in a million years. Sure it looks cool but it has a number of problems.

1) How do you get a heater and filter in?
2) How do you clean it? Inside and out.
3) You could never use it. The water temp in the sink bowl would change the water temp in the tank and would kill the fish.
4) For Bettas: No air space at the top nor air flow. Come to think of it the lack of air exchange surface would affect all fish.

Sorry but no, not in a million years. Sure it looks cool but it has a number of problems.

1) How do you get a heater and filter in?
2) How do you clean it? Inside and out.
3) You could never use it. The water temp in the sink bowl would change the water temp in the tank and would kill the fish.
4) For Bettas: No air space at the top nor air flow. Come to think of it the lack of air exchange surface would affect all fish.

How do you scrub the glass when the algae builds up? That begs another question, where does it get light when the bathroom lights are off. Few bathrooms have windows and if they do they are usually frosted or have blinds over them. Putting a lighted hood on it would defeat the purpose.

How do you Vacuum the corners, the front or the back? You can only get at the gravel right under the plugs...

Don't get me wrong, I wish all furniture could be a fish tank for me to keep bettas in. Then logic steps in and dashes all my dreams. Sorry again.

Edit: Just thought of another point. As soon as you ran hot or cold water in the sink the glass would likely shatter from the extreme temp difference.

maybe one could modify it, so that the top glass comes off. not the entire bowl section, just the top. it'd be neat, and cooler than a fish tank toilet. >.>;

Yeah I'm in agreement on the fish tank toilet. Really why traumatize the fish? (I see it this way: Why put a goldfish so close to it's possible grave? I'd never do that to my betta's hence why I say goldfish.)

How about this... it may look cool, but in the picture, in the upper right bubble there's a betta shown... can you imagine the poor betta wandering down one of those tubes and freaking out because it can't get to the surface? O.O